It is generally accepted that arterial baroreflex control and vagal control of heart rate is diminished with increases in age (>60 years) in the human. These phenomena have been used to explain the increased incidence of orthostatic hypotension observed in the healthy older population. More recently, however, we have documented that discrete carotid baroreflex control of the heart rate of healthy individuals 60-69 years of age was no different than that of healthy individuals 18-36 years of age. Therefore, we hypothesize, that the discrete aortic baroreceptor mediated reflex control of heart rate of the older individuals must be compromised. Other findings suggest that as individuals age the augmentation of the carotid-cardiac reflex control of heart rate by central hypovolemia, observed in young adults is diminished. Therefore, we further hypothesize that the interactions of carotid baroreflex function with the status of central blood volume (hypovolemia or hypervolemia) is diminished in older subjects (60-75 years). To test these hypotheses, we will isolate the carotid and aortic baroreflex control of heart rate during euvolemic, hypovolemic and hypervolemic central blood volumes, in a cross-sectional investigation of 18-36 year old adults and 60-75 year old adults of both genders. In addition, examination of the vagal control of heart rate using power spectral analysis will be made across all conditions and age groups. It is anticipated that the data obtained will provide us insight into the mechanism of altered arterial baroreflex control of heart rate of the elderly during orthostasis. Furthermore, we expect to develop a model of orthostatic hypotension upon which further investigations into the mechanisms of the age related increase in orthostatic hypotension can be made. Our long-term objectives are to understand the mechanisms of aging on human neurohumoral regulation of blood volume and blood pressure and to provide the basis for the use of prophylactic measures to enhance the quality of life of the elderly population.